In cellular telephones systems, the communication from the base station to the mobile station, such as a handheld radiotelephone, is carried out over a forward channel; and, conversely, the communication from the mobile station to the base station is carried out over a reverse channel. Under the Interim Standard IS-95-A, which has been adopted by the Telecommunications Industry Association for implementing CDMA, pages are sent from the base station to a mobile station over a forward channel referred to as a paging channel. The page informs the mobile station that a call has been placed to it.
IS-95-A provides for a slotted mode feature, which allows a mobile station to operate in a reduced power mode. That is, the paging channel is divided into 80 millisecond (ms) intervals called paging channel slots, and each mobile station operating in the slotted mode is assigned a specific slot in which they monitor the paging channel. The assigned slot is periodic-called a slot cycle. Because the mobile station only needs to monitor the paging channel during its assigned slot, at all other times of the slot cycle the mobile station can "sleep," that is, go into the reduced power mode by, for example, turning off its radio frequency (RF) portion. To monitor the next periodic slot, the mobile station must "wake up" from its sleep in time to monitor the paging channel at the assigned slot.
The cellular system comprises a number of base stations scattered over a geographic area, and the mobile station can move throughout the area. Before the mobile station can monitor the paging channel after waking, it must first determine from which base station it will monitor the paging channel, preferably the base station that provides the strongest channel signal. In the CDMA system according to IS-95-A, each base station continuously transmits a pilot channel signal. The pilot signal transmitted by each base station has the same spreading code but with a different code phase offset. Phase offset allows the pilot signals to be distinguished from one another, which in turn allows the base stations to be distinguished. IS-95 defines the spreading code as a PN sequence having a period 2.sup.15 chips, and phase offset as a multiple of 64 chips relative a zero offset pilot PN sequence.
In a static environment, the mobile station stays within the coverage of the base station it was monitoring before going to sleep (called an "active pilot"), and, when it wakes up, will most likely acquire the same pilot signal it monitored before going to sleep. In a dynamic environment, the mobile station may not be in the coverage of the base station it was monitoring before sleeping, and a handoff to another base station may be necessary before the paging channel can be monitored. The pilot channel acquisition--whether of the active pilot or of another base station--must occur before the assigned slot is to be monitored so that the mobile station does not miss any pages and, consequently, not miss any calls.
A need therefore exists for a method of and apparatus for acquiring a pilot channel prior to the arrival of the assigned slot when the mobile station is operating in the slotted mode of a CDMA system.